A significant consideration in the design of an aircraft is the effect on lightning strike. When most of the airframe structure is metallic, lightning current is carried by the primary structure. Since the metallic structure generally has large cross sectional areas, lightning currents are thus not considered to have a significant effect on fuel, hydraulic, electrical and utility duct sub-systems. Fibre-reinforced composites are increasingly commonly used for many aspects of airframe design and manufacture, including structural elements. With composite structures the resistance of the primary structure is significantly higher than an equivalent metallic structure resulting in the traditional metallic tubing or duct systems being the most conductive element in the airframe. In many applications, such as fuel, venting or inerting systems, where flammable fluids or vapours can be present in a pipe run. Thus any electrical arcing resulting from a lightening strike within or around the pipe run will be potentially hazardous and thus must be minimised.
For a typical fuel system made of pipe connectors and pipes, there are two fundamental design approaches to avoid this problem. The first is to use short non-metallic isolator or resistor sections in a largely metallic tubing system to electrically separate the lengths of metallic tubing. An insulating section of pipe may be formed from a polymer such as nylon.
The second approach is to replace part of the metallic tubing system with longer non-metallic sections. In that case, the static charge accumulated on the non-metallic components need to be discharge. The non-metallic section is made of highly resistive material which allows static discharge while limiting lightning current to flow though it. A highly resistive section of pipe may be formed from a composite material loaded with conductive fillers. While it is technically possible to form the entire tubing system from highly resistive material, due to the application of conventional manufacturing and stress approaches, this is not a practical solution and would in any case carry a high cost.
A problem with both if these approaches are that the insulators or isolators are expensive and relatively heavy.